When you’re evaluating cylinder heads to use for a build, what performance numbers are you looking at? Most people are probably only looking at the peak flow numbers for a set of heads, and while this is a good number, it’s not the most important. You want to know what the low- and mid-lift flow numbers are for a set of heads, and in this article, you’ll learn why those are the most critical numbers.
Cylinder heads are the lungs of an engine — they help bring the air into the combustion chamber and expel it through the exhaust system. The amount of air a cylinder head can flow is measured in cubic feet per minute (CFM), so the larger the CFM number, the more air a set of heads can flow. A healthy peak CFM number is a good thing, but it’s the low- and mid-lift flow numbers that really define how well a cylinder head will work.
Alex George from Air Flow Research (AFR) explains what these numbers, known as the “area under the curve,” are all about.
“Valve lift is the distance by which the valve is raised from its seated position when it’s fully opened. Measuring airflow through a valve port is plotted on a graph for the entire valve cycle from opening to peak lift and back to close. This is what we like to refer to as ‘area under the curve’ when it’s related to cylinder head airflow figures, which in turn corresponds to total power. Mid-lift flow numbers are about 60-percent of max lift of the cam and of where the port stops flowing.”
So, why are these numbers so important? The biggest reason is because of how long the lifter will be in this region. In most cases, the lifter will be in this area twice and for an extended period of time. According to George, the low lift numbers are usually .100-.300 and are critical to getting good air and fuel velocity. It’s very important to get that first rush of air and fuel moving quickly so the rest of the mixture can follow it into the cylinder.
“Racers should look for as close to 80-90 percent of peak flow when at 60-percent of peak flow, with as good as off-the-seat numbers as you can get with a standard valve angle non-raised runner racing head. Engine builders who can fatten the curve on both sides of peak lift in the low to mid-range area, rather than just raising the maximum peak, will end up with more total area under the curve and more usable power. Contrary to popular belief, a head with strong low and mid-lift flow doesn’t mean it will have stronger power in the lower and middle part of the horsepower curve, although it will make more there. It means it will make more power everywhere due to the fact that every time an intake and exhaust valve opens and closes, it will be able to move more air in total than a head with similar peak numbers and a much softer area under the curve,” George says.
The low and mid-lift numbers are impacted by how efficient a cylinder head works. A head that can move air without major effort is going to have much better low and mid-lift numbers, thus improving the overall performance of the cylinder head. AFR keeps this in mind when it’s designing cylinder heads, and pays close attention to the port design and the valve job that’s performed on the heads.
“An efficient head with better low- and mid-lift flow numbers usually shows its stuff up top, allowing the torque curve to hang on a little longer, producing a higher peak horsepower figure, and does not roll over as quickly past peak flow. That’s because at higher RPM, when there is much less time to fill and evacuate the cylinder, the airflow demand of the engine is at a premium, and any little extra you cram in there (and remove) ends up paying big dividends. NHRA Stock Eliminator racers for example are very keen on optimizing average flow instead of peak flow. On a stocker, you might be able to get good some solid numbers past .500-inch lift, but most of them, if not all, have to work below that number. More often than not, you will find a cylinder that has the highest average will outperform the cylinder head with the higher peak lift numbers,” George states.
You can learn more about how and mid-lift flow numbers right here on AFR’s website.